Three Christmases ago, the Guardian moved from Farringdon Road to a patch of regenerating edgeland north-east of Kings Cross. And, ever since, there's been an unbeatable new boon to working here: you're never more than a hop or a skip from where they shot The Ladykillers.

Alexander Mackendrick's 1955 comedy is Ealing's neatest, and its trippiest; the product of lurid new colour stock (including some alarming back-projection) and a hallucinatory premise. The plot – five faintly spivvy crims, headed up by a bafflingly dastardly Alec Guinness, get an old lady to aid them in a bullion heist, then (spoiler alert) one by one die in their efforts to bump her off – apparently came, fully formed, to writer William Rose in a dream.

Rose fell out with Mackendrick in pre-production, so the director pieced together the script from Rose's notes (when the writer saw the film later, he thought they'd improved on his vision). Could that explain the snap and crackle? The quick spin and nifty lick that makes it feel more akin to The Sweet Smell of Success than to creaky Brit-coms of the same vintage.

For what elevates The Ladykillers way above panto predictability is that it operates slightly off-centre; it takes its cue from its heroine (christened Mrs Lopsided), rattling about in her wonky house, perched by the railway sidings. What makes it funny is not that they employ her unwitting services, but that she's so sweet-natured about it (the cops even give her a lift back with the cash). That, when the penny drops, fear never crosses her mind, only opprobrium. And that her reprimands are effective: some of the gang are admonished, genuinely shamed. Even the most dark-hearted can't even manage to wring her neck.

The Ladykillers Photograph: Ronald Grant Archive

It's a film with jazz in its bones and rhythm to its beats: I love the sudden violence with which Mrs Wilberforce mallets the cold water pipe, the lovely nod to The Lodger when Alec Guinness first materialises at the house, silhouetted behind the frosted glass. The audio gags – a bleed from parrot yelps to train-tunnel screams, the gulped asides and the fade-out lines ("It's a brown horse, 11 years old, answers to the name of Dennis …").

Like my other favourite, I can't remember the first time I saw it. There was no epiphany or rite of passage, no lucky stumble into the wrong cinema. It's just always been there. It was one of three VHS tapes bought on merit when I was growing up. And as someone who spent a lot of time with their grandmother, it seemed only natural that bank robbers would meet their match in a benevolent pensioner.

The film itself is a little deeper: as Mackendrick explained in a book published 2005, Mrs W, with her nods to her Navy husband, and her aged friends, is upstanding Old Britiain – conservative sterness rapping the fingers of economic innovation.

The fable of The Ladykillers is a comic and ironic joke about the condition of postwar England. After the war, the country was going through a kind of quiet, typically British but nevertheless historically fundamental revolution. Though few people were prepared to face up to it, the great days of the Empire were gone for ever. British society was shattered with the same kind of conflicts appearing in many other countries: an impoverished and disillusioned upper class, a brutalised working class, juvenile delinquency among the mods and rockers, an influx of foreign and potentially criminal elements, and a collapse of "intellectual" leadership. All of these threatened the stability of the national character.

Though at no time did Bill Rose or I ever spell this out, look at the characters in the film. The Major (played by Cecil Parker), a conman, is a caricature of the decadent military ruling class. One Round (Danny Green) is the oafish representative of the British masses. Harry (Peter Sellers) is the spiv, the worthless younger generation. Louis (Herbert Lorn) is the dangerously unassimilated foreigner. They are a composite cartoon of Britain's corruption. The tiny figure of Mrs Wilberforce (Wilberforce was the name of the 19th-century idealist who called for the abolition of slavery) is plainly a much diminished Britannia. Her house is in a cul-de-sac. Shabby and cluttered with memories of the days when Britain's navy ruled the world and captains gallantly stayed on the bridge as their ship went down, her house is structurally unsound. Dwarfed by the grim landscape of railway yards and screaming express trains, it is Edwardian England, an anachronism in the contemporary world.

Bill Rose's sentimental hope for the country that he and I saw through fond but sceptical eyes was that it might still, against all logic, survive its enemies. A theme, a message of sorts, one that I felt very attached to. But one that it took quite some time for me to consciously recognise and appreciate.

The Ladykillers Photograph: Alamy

I didn't twig that at the time, of course. And today I still love it not for its ambivalent social message but because it reminds me of my granny, and the teas and the friends she used to have, because of the rackety parrots and the fabulous RP station announcements. The foggy menace of that final shootout, the exotic struts of the gaswork, the intricate structure of the screenplay that gives us that beautiful bookend payoff, in which the cops write her off as dotty and so urge her to keep the lolly. I can think of no happier ending than that one, in which Katie Johnson totters down the street, ignoring her brolly, rewarding the tramp, as the sun rises over St Pancras and the trains hoot below, and the five coshed corpses chug their way north.